1. Field of the Invention
The present invention relates to an image forming apparatus, such as copiers and printers, and specifically relates to a toner used in an image forming apparatus having at least an image bearing member, a charging unit, a developing unit, a transfer unit, a cleaning unit, a lubricant applying unit containing a lubricant.
2. Description of the Related Art
FIGS. 1 and 2 are respectively an illustration showing the entire configuration of a conventional image forming apparatus.
The image forming apparatus forms an image by charging uniformly an image forming area on an image bearing member by means of a charging unit, by writing the image on the image bearing member by means of an exposing unit, and using a toner frictionally charged on the image bearing member by means of a developing unit. Then, the image on the image bearing member is transferred onto a printing paper by means of a transfer unit directly onto the paper fed from a paper feeding unit or indirectly via an intermediate transfer member, after that the image is fixed onto the printing paper by means of a fixing unit.
Meanwhile, a residual untransferred toner remaining on the image bearing member is wiped off from the image bearing member by a cleaning unit. The image bearing member, being cylindrically-shaped or belt-shaped, has gone through a series of the image forming process steps and then enters into the next image forming process.
There are two systems of image forming apparatus involving such processes as described above. One is a revolver system in which a single image bearing member is present and an image is formed on the single image bearing member for every color. And the other is a tandem system in which an image bearing members is used for each color. The revolver system costs less. The tandem system costs high but allows for high-speed printing. At present, tandem system image forming apparatuses capable of printing at high speeds are mainly used.
When the tandem system is used, a formed image is primarily transferred onto an intermediate transfer member, all colors are superimposed on the intermediate transfer member, and then a full-color image with the all colors superimposed thereon is transferred to a printing paper by a secondary transfer unit.
The following is a description on the units used in each process of the conventional image forming apparatus.
<Charging Unit>
Examples of a charging unit (1) include a proximate charging system and contact charging system each using DC or DC overlapped with AC, and corona charging system. Examples of the corona charging system include corotron chargers and scorotron chargers.
As a charging unit charging an image bearing member, a corotron charger and a scorotron charger using a corona discharge have been mainly used so far. However, a charging unit (1) using a corona discharge has drawbacks that a large quantity of ozone is produced, and NOx, etc., produced by the corona discharge, adhere to the image bearing member which causes problems with an image deletion with time. Furthermore, to generate a corona discharge, a high voltage power source for applying a voltage of 5 kV to 10 kV was required. Therefore it is difficult to reduce the cost of the image forming apparatus.
To solve such a problem, in recent years, as a charging unit that can be applied to an image forming apparatus, there have been a variety of charging units proposed, such as a contact type charging unit that makes contact with an image bearing member without using a corona discharge, and a proximate type charging unit in which a charging unit is placed closely to an image bearing member. The contact type/proximate type charging units can solve above many drawbacks noted for a charging unit using corona discharge, however they cause such a problem that a wear amount of the image bearing member increases, which shortens the lifetime of the image bearing member. Furthermore, occurrence of noise is also a drawback when an alternating current is used for voltage applied. In addition, since a charging unit (1) rubs against an image bearing member with a toner or paper powder, the surface of the image bearing member is further contaminated and a drawback due to contamination of charging unit surface occurs.
<Exposing Unit>
Examples of an exposing unit (2) include such an exposing unit as using LD, LED lamps, and xenone lamps.
<Developing Unit>
Examples of a developing unit (3) include a one-component development unit (3) and a two-component development unit using a mixture of a toner and carrier.
Developers are classified into two types of a two-component developer composed of a toner and carrier and a one-component developer composed of a magnetic toner or nonmagnetic toner. Generally, these toners are manufactured by a kneading pulverization method in which a resin, pigment, charge controlling agent, and releasing agent are cooled and then pulverized and classified. However, this method causes nonuniform particle diameters and nonuniform particle shapes of the toners and they are difficult to be controlled.
In such circumstance in recent years, there is a trend to intensionally control the particle diameter of a toner and solve the above-mentioned drawbacks. And as a granulation method in an aqueous system, toner polymerization methods such as an emulsification polymerization method and dissolution suspension method have become increasingly used.
In recent years, a high image quality is increasingly requested, especially in color image formation, in order for highly fine images to be put into practice, a toner having a reduced particle diameter and similar particle diameters are increasingly requested. When an image is formed using a toner with a widely dispersed distribution of particle diameter, it substantially causes drawbacks that a fine powder toner contaminates a developing sleeve, contact/proximate charging unit, cleaning blade, photoconductor, and carrier, and causes toner scattering, which make it difficult to satisfy both high image quality and high reliability. Meanwhile, when a toner with similar particle diameters and a sharp distribution of particle diameter is used, minute dot reproducibility is greatly improved because of its uniform developing behavior of individual toner particles.
However, when a toner with reduced particle diameters and similar particle diameters is used, problems with cleaning ability arise. In particular, in blade cleaning it is impossible to remove a toner having similar and reduced particle diameters stably. To solve the problem, various methods to improve cleaning ability are proposed by using an improved toner. Among these methods, a method is present in which a toner is deformed from a spherically shaped toner to improve the cleaning ability. This method enables to block flow of the toner much easily by a blade cleaning, through deforming the toner shape with concomitant decrease in flowability of a fine particle toner. Note, however, that when a toner is deformed to too much degree minute dot reproducibility degrades with unstable behaviors of the toner in developing step. In this way since properties of a toner such as transfer quality, transfer efficiency, and cleaning ability are influenced by toner shape, it is required to optimally design the distribution of toner shape in order to obtain a toner having above described properties.
<Transfer Unit>
Examples of a transfer unit (4) include transfer units using a transfer belt, transfer charger, and transfer roller.
<Cleaning Unit>
Examples of a cleaning unit (7) include a blade-shape cleaning blade composed of polyurethane rubber, silicone rubber, nitrile rubber, chloroprene rubber and so forth, or a fur brush, elastic roller, roller covered with a tube, nonwoven cloth, and so forth.
So far, a cleaning method using a blade was mainly used for cleaning in image forming apparatuses using electrophotography and there have been many image forming apparatuses having only cleaning units (7) of blades. In addition some high-speed machines are equipped with a cleaning support unit (11) in order to avoid a situation in which a large amount of the toner adhere partially.
In this case, when a cleaning blade is used in a cleaning unit, the cleaning blade has contact with the image bearing members in a trailing direction or in a counter direction.
<Cleaning Support Unit>
When usage of cleaning units alone results in insufficient removal of an untransferred residual toner remaining on an image bearing member, commonly a cleaning support unit is placed downstream with respect to the rotational direction of the image bearing member, upstream to a cleaning unit, thereby cleaning ability is improved.
Examples of a cleaning support unit include a fur brush, elastic roller, roller covered with a tube, and unwoven cloth.
Conventionally, a cleaning support unit is placed upstream to the cleaning unit, and examples thereof include above described members. The method aims to improve cleaning ability by disturbing mechanically a toner which enters a cleaning unit using a cleaning unit.
An image forming apparatus is also commercialized in which a voltage is applied to a cleaning support unit at this time and the cleaning ability is enhanced through control of polarity of the toner.
<Aqueous Granulation Toner>
In such image forming apparatuses as described above, an aqueous granulation toner is desired to be used in order to obtain images with high quality. Specifically, a technology to manufacture a spherical toner in a wet process by suspension polymerization or emulsification polymerization method (Japanese Patent Application Laid-Open (JP-A) No. 01-257857), and a technology to conglobate a pulverized toner by hearing (Japanese Patent Application Publication (JP-B) No. 04-27897 and Japanese Patent Application Laid-Open (JP-A) No. 06-317928) are proposed. According to these toner production methods, a toner is easily reduced in particle diameter.
<Lubricant Applying Unit>
Use of an aqueous granulation toner makes it difficult to maintain cleaning ability level. Consequently, when a highly spherical toner is used, an unit is often placed to coat the image bearing member with a lubricant to improve cleaning ability margin, or to prevent the wear of the image bearing member due to an electric discharge from the charging unit, the wear and filming of the image bearing member due to a contact with the cleaning unit or toner.
Examples of a lubricant applying unit may include units to coat the image bearing member by using a fur brush or loop brush, roller, and belt or units to coat directly the image bearing member with a solid lubricant or with a powder of a lubricant.
<Lubricant Applying Method>
As a method to coat an image bearing member with the lubricant as described above, a technology was used, in which a lubricant was externally added to a toner and the image bearing member was coated with the lubricant with a supply of the toner. However, this technology could not prevent the image bearing member from wear due to discharge and due to a contact with members, because areas on the image bearing member where no toner was supplied (a non-image area) were not coated with the lubricant.
In addition, another technology was also used, in which a solid lubricant was directly contacted with a cleaning support unit and an image bearing member was coated with the lubricant thereby. However, this technology could not prevent the image bearing member from wear due to discharge and to contact with members, because areas on the image bearing member where an untransferred residual toner was present (image area), were not coated with the lubricant.
To solve these problems, a method was proposed, in which an image bearing member was directly contacted with a powder lubricant downstream to the cleaning unit with respect to the rotational direction of the image bearing member, further a smoothing blade for the lubricant was placed downstream with respect to the rotational direction of the image bearing member and upstream to the charging unit, and the whole surface of the image bearing member was coated with the lubricant thereby. In addition, as a similar method which enables to coat uniformly with a lubricant, another method was also proposed, in which a lubricant applying unit was pressed with a lubricant downstream to the cleaning unit with respect to the rotational direction of the image bearing member, the image bearing member was coated with the lubricant by means of the lubricant applying units, further a smoothing blade for the lubricant was placed downstream with respect to the rotational direction of the image bearing member and upstream to the charging unit, and the whole surface of the image bearing member were coated with the lubricant thereby. By using these methods, it became possible to coat the whole surface of an image bearing member with a lubricant and to protect the whole surface of the image bearing member from wear due to the electric discharge from the charging unit or due to the contact with the members.
<Lubricant>
In order to extend the lifetime and to promote image quality, some examples to coat an image bearing member with a lubricant are known. The reason why a lubricant is supplied over a surface of an image bearing member is to solve the following two problems.                (i) to prevent a generation of toner filming (fusion)        (ii) to improve a transfer efficiency by lowering a friction coefficient and prevent an occurrence of cleaning defects        
To solve these problems disclosed methods (for example, JP-A Nos. 2002-244516, 2002-156877, 2002-55580, and 2002-244487) are known and the problems are solved by coating an image bearing member (8) with a lubricant (5). In all the cases of these examples, the problems are solved by coating an image bearing member (8) with a lubricant (5) and by lowering the friction coefficient.
Furthermore, JP-A No. 2002-229227 discloses an example, in which in order to extend the lifetime of a charging unit and image bearing member, a noncontact charging unit is used, an inorganic particle is dispersed in a photosensitive layer of the image bearing member, and the wear resistance is improved by coating with such a lubricant as zinc stearate.
In addition, another example of image forming apparatus is present, which has a blade-shape auxiliary member configured to attach a lubricant thinly and uniformly between the charging unit and the developing unit the image bearing member and to block lubricant particles having large diameters (refer to JP-A No. 10-142897).
Examples of lubricants used include fluorochemical resins in a powder form, solid form, and film form of (such as polytetrafluoroethylene, polyvinylidene-fluoride), metal fatty acid salt having a lamella crystal structure such as zinc stearate, magnesium stearate, and calcium stearate (the other examples include lauroyl lysine, sodium zinc salt of monocetyl phosphate ester, and calcium lauroyl taurine), liquid materials such as silicone oils and fluorochemical oils, natural waxes, and synthetic waxes, and gaseous materials. Each of these lubricants is externally added and reacted.
<Applied Amount of Lubricant>
As an applied amount of a lubricant over a surface of an image bearing member at that time, an optimal applied amount is proposed as follows (refer to JP-A No. 2005-17469).
An optimal amount of a lubricant is determined so that a percentage of the number of specific elements of the lubricant materials detected by an X-ray photoelectron spectrometer (XPS) to the sum of the number of all elements of materials constituting the outermost surface of the charged body detected by XPS is set to a value equal to or more than a value calculated by the following Expression (1).1.52×10−4×{Vpp−2×Vth}×f/v×Nα  Expression (1)
(Wherein, “Vpp” is a peak-to-peak voltage value (V) of AC voltage, “f” is a frequency (Hz) of alternating current component applied to the charging unit (1), “v” is a moving speed (mm/sec) of the surface of the charged body, “Nα” is the number of specific elements in a molecule of the lubricant material. And “Vth” is a sparkover voltage and calculated using the following Expression (2).Vth=312+6.2×(d/εopc+Gp/εair)+(7737.6×d/εopc)1/2  Expression (2)
(Wherein, “d” is a membrane thickness (μm) of the charged body, “εopc” is a specific inductive capacity of the charged body, “εair” is a specific inductive capacity of space between the charged body and the charging unit, “Gp” is the minimum distance (μm) between the surface of the charging unit (1) and the surface of the charged body.
By above described invention concerning applying method of a lubricant, the whole surface of an image bearing member could be coated uniformly with a lubricant, abrasion wear of the image bearing member due to an electric discharge from the charging unit could be reduced, and the lifetime of the image bearing member could be extended thereby.
However it was found that thin line reproducibility is remarkably degraded, when an aqueous granulation toner with its volume average particle diameter (Dv) being in the range of 3.0 μm<Dv<6.5 μm, containing at least a binder resin, colorant, and laminar inorganic mineral in which at least part of a metal cation is modified with an organic ion, is used for printing.
Usually when a thin line is output as an image, a toner layer is formed thick in the central part of the thin line, and when the image is transferred onto an intermediate transfer member or onto a printing paper, a transfer pressure could not be evenly applied to the toner layer, the transfer pressure is concentrated on the central part of the thin line where the toner layer is thick. Consequently, the toner layer is packed and flocculated strongly in the central part of the thin line where the transfer pressure is concentrated.
At this time, when an electrostatic or nonelectrostatic adhesion force to the object on which the toner image was formed before the transfer is high, the whole of the toner layer can not be transferred at the central part of the thin line where the toner layer strongly flocculates, which results in a transfer defect (missing the central part of thin line).
Furthermore, this phenomenon tends to take place in a situation when developer is stirred for long time in an environment of few replacements of a toner in a development container and the toner degrades with time. As a result flowability of the toner become poor. And above described poor transfer (missing the central part of thin line) is considered to tend to take place in above situations because of an increase in nonelectrostatic adhesion force of the toner and easy flocculation of the toner layer.